[time-nuts] LPRO-101 with Brooks Shera's GPS locking circuit

[Brooks Shera]

----- Original Message ----- 
From: "Ulrich Bangert" <df6jb at ulrich-bangert.de>
To: "'Discussion of precise time and frequency measurement'" 
<time-nuts at febo.com>
Sent: Friday, December 15, 2006 05:47
Subject: Re: [time-nuts] LPRO-101 with Brooks Shera's GPS locking circuit


.......
>I second Bruces's opinion about what is an overshot or not. When ps
>reolution is ready available then why not use it? I attach a online
>output from my DIY GPSDO from a few minutes ago that shows the M12+'s
>signal properties when measured with abt. 110 ps resolution against a
>FTS1200. The yellow line reperesents a prefiltered version of the
>sawtooth corrected values (blue). The filter time constant is 1/3 of the
>loop time constant as in a PRS-10. The yellow values are the ones to
>feed the regulation loop.

>What I wanted to explain is the Shera concept noise floor is a factor 20
>above what a modern receiver can deliver (again inc. the sawtoth
>correction). And yes, you are right: There were different numbers when
>this concept was thought out! And exactly because different number were
>there when this concept was thougt out I am going to ask why people
>still built it today.

>Best regards
>Ulrich Bangert, DF6JB


I believe the sawtooth correction is of little or no value for a GPSDO, 
which typically requires a low pass filter between the GPS 1pps and the 
disciplined oscillator.  This filter is quite effective in removing the 
sawtooth quantization introduced by the GPS rcvr clock, just as it removes 
the similiar quantization caused by my phase detector.

For example, reading from your graph I averaged the raw data (as best I 
could by reading the blue line).  The running average of the raw data over 
40 sec (starting at 12:31:30) was -4.5 nsec, after 60 sec it was -4.2 nsec. 
These values appear to be indistinguishable from the values you get by 
averaging the "sawtooth corrected" data (the yellow line).

It appears from your plot that the sawtooth correction has contributed very 
little or nothing that averaging does not already provide.   Have I 
misunderstand something?

I believe that your "noise floor is a factor 20 above what a modern receiver 
can deliver" statement is incorrect.  With an HP 5720B (about 100 psec 
resolution), I have measured the phase difference between the GPS 1pps and 
the phase of a 5 MHz oscillator controlled by my controller. This data has 
been compared with simultaneous phase serial output from the controller as 
determined its maligned 24 MHz asynchronous internal phase measurement 
circuitry.

ADEV Stable 32 plots of both data sets are essentially identical.  From this 
I conclude that nothing would be gained, for the purpose of discipling an 
oscillator, by using a more elaborate and expensive phase detector  (the 
phase detector in my controller costs $6.61, including $5.35 for the dual 24 
MHz osc that is shared as the PIC clock).  It was my goal when I designed 
the controller was to make the design transparent to the builder and to use 
as few parts as necessary consistant with performance limited only by 
available GPS receivers and VCXOs.  When I wrote the QST article I was 
totally ignorant of the fact that I could buy the HP58503 with similiar 
performance for $5400.

Your earlier comment about the capture range of the phase detector is well 
taken.  For the past several years the PIC software I provide has included 
an option designed for use with inexpensive TCVCXOs.  It requires only an 
external 128 divider chip and produces EFC voltages suitable for inexpensive 
oscillators.  It works very well and provides sufficient performance for 
many applications.

Regards,  Brooks





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